This invention relates in general to interconnects for use in communication networks, particularly in high speed computer backplanes. More specifically, it relates to an optical interconnect operating in a wavelength division multiple access architecture.
In computer technology there has been considerable growth in recent years in parallel processing, that is, multiple processors operating in parallel rather than one processor operating serially. To operate efficiently, the processors and memory must be interconnected in a way that allows a very high speed flow of data among the processors along any path, and with a low latency (latency being measured by M/B where M is the bytes to be routed through the system and B is the bandwidth of the channel in bytes per second).
Time division multiplexing (TDM) and a variety of architectures have been proposed. However, TDM is effective for only a limited number of processors. If the system must be scaled up, the bus connecting the processors is soon saturated. Various time shared buses have been investigated to scale up. FIG. 1A shows a hierarchy bus architecture. FIGS. 1B, 1C and 1D show ring, mesh and hypercube architectures to interconnect processors without the limitations of bus-based architectures. PE in these figures denotes a processor element; I/O denotes an input/output interface. These non-bus systems, however, offer only limited scaling since data routed between non-adjacent processors must pass through several processors. This situation is characterized by large node delays and high system latency. Moreover, all of these architectures exhibit speed and distance constraints due to electromagnetic effects such as inductive reactance to fast rising signals. The result is that electrical signals are transmitted over very short lengths (inches) at high speed data transmission rates (100's of Megabits per second), or speed is compromised for length. However, recent research has indicated that regardless of the architecture or routing, the speed of solution of a problem by multiple processors is ultimately limited by the communication latency, regardless of the number of processors applied to the problem. Attempts to interconnect large arrays of processors has also been hampered by power consumption considerations.
In communications networks, optical connections between processors are well known. TDM systems are most common, but wavelength division multiple access (WDMA) network implementations are also known. The WDMA systems are traditionally based on a passive star architecture as shown generally in FIG. 2. Communications between any two processors in the network is established over a common wavelength channel between two elements. Assuming only one transmitter attempts to communicate simultaneously with any one receiver, "n" processors can establish "n" asynchronous communications links operating simultaneously. This system also has the ability to establish transmit/receive wavelength channels that are unique and non-interfering from channel to channel. Broad spectrum bandwidth LED's (30-150 nm) have used the selective filtering characteristics of WDM devices to provide a number of different slices from identical LED sources. This arrangement multiplexes a number of communicating signals over a single fiber optic transmission line. This arrangement however, has proven to be severely power limited due to the power loss in a given channel due to the spectral slicing process. Dispersion is another source of power loss. These power loss problems have limited this technology to relatively short link lengths or low data rate applications.
To address one or more processors to receive a transmission from a transmitting processor, it is known (i) to fix the transmitters at a selected frequency and tune the receivers over the operating bandwidth, (ii) to fix the frequency of the receivers and to tune the transmitters, or (iii) to tune filters disposed between fixed frequency transmitters and fixed frequency receivers.
Several WDMA networks have been implemented using a fixed source, variable receiver approach. One is AT&T's Lambdanet. It uses a laser source at each processor which operates at a fixed wavelength. Outputs from each processor are combined at a star coupler. A wavelength division demultiplexer (WDDM) at each processor decomposes the combined optical spectrum from the star coupler into each of the applied input wavelengths by spectral filtering. Each output leg of the WDDM is applied to a separate receiver. Therefore at each processor there must be a laser source (operating at a unique wavelength), a WDDM, and "n" optical receivers. This arrangement is very costly since (i) optical receivers are costly and (ii) to support "n" channels there must be n.sup.2 receivers. Also, the laser sources are costly.
A second WDMA implementation by IBM called Rainbow I also uses a laser transmit source operating at a wavelength uniquely associated with that source. Connection is established via one photodetector at each processor preceded by a Fabry-Perot filter package. The transmitter continuously transmits a broadcast request specifying a destination receiver. If a receiver is not occupied, it continuously searches for a request by tuning the associated spectral filter. When the designated receiver detects a request for transmission to itself, a link is established. One drawback with this system is the loss of time inherent in the system while the spectral filter sweeps through the bandwidth to make a link. Another is the cost of the tunable filters. Further, both Lambdanet and Rainbow I rely on optical sources that operate at a well-defined wavelength, despite temperature variations and changes in other operation parameters. Lasers with this capability require unusually good quality control during manufacture and are comparatively expensive.
To produce a system that varies the wavelength of the transmitter requires a reliably tunable laser. With conventional lasers it is necessary to change the chemistry from batch to batch to produce a series of devices at varying wavelengths. Laser devices can also be tuned over limited ranges by temperature control, but this approach is costly and has a large power consumption. Recent research on a tunable laser has focused on additional external cavities that can be selected to produce a given wavelength. At present, however, such tunable lasers are not commercially available. Electronically tunable lasers are known and commercially available, but they can be tuned over only a few nanometers range. Also, it is not possible to change the frequency of the sources fast enough for high speed backplane applications or many communication network applications.
Other light sources besides lasers are known, but heretofore the principal alternative light source has been the light-emitting diode (LED). It, however, is not a coherent source. As such, as noted above in the discussion of prior spectral slicing links, it requires more power to compensate for the effects of chromatic dispersion when its light output is transmitted along an optical fiber. In addition, LED's are known as having a comparatively low data transmission rate and as being not tunable to a unique frequency, or tunable within a range of frequencies.
It is therefore a principal object of the present invention to provide an interconnect for the transmission of data that operates at a sufficiently high speed and with a sufficiently low latency to interconnect backplanes of parallel processors.
Another principal object is to provide the foregoing advantages at a low cost using no tunable sources, receivers or filters and not requiring high cost lasers with a well defined wavelength output.
A further object is to provide the foregoing advantages while exhibiting an acceptable power loss and low cross talk between channels.
Yet another object is to provide an interconnect with the foregoing advantages that is not limited by transmission line effects and allows transmissions over multiple channels at throughputs of up to 3.2 Gigabits per second (Gbps).
A further object is to provide an interconnect that is not limited to any given protocol and which can operate in a variety of applications, including the interconnection of the backplanes of a large array of parallel processors and the replacement of a conventional backplane servicing multiple processors connected to a common bus.
A still further object is to provide an interconnect which allows the simultaneous interconnect between multiple processors in either an addressee-specific mode or a broadcast mode.
Yet another object is to provide an interconnect which can be reconfigured.
Another object is to provide an interconnect with the foregoing advantages which is substantially insensitive to temperature variations and electromagnetic interference.
Still another object is to provide an interconnect with the foregoing advantages that can be formed using known, comparatively low cost components.